This invention relates to a circuit for allowing a phase sensitive circuit to switch between different reference signals and more particularly to such a circuit which operates without requiring separate input phase locked loops.
The problem of controlling phase sensitive circuits when switching between different input reference signals, even when the signals have the same frequency, is well-known. The problem arises from the possible phase difference between the signals. Thus, when a switch is made to the "other" reference input and that input is even slightly out of phase with the "original" reference input, circuitry of the main circuit detects the difference and immediately readjusts the main circuit to be in synchronization with the new input. At best this is disruptive to the operation of the circuit. When the main circuit provides reference signals for subsequent data transfer, as in a PBX, severe data errors occur with each such switch of reference inputs.
Typically, the problem is overcome by forcing the input signals to have the same phase. This is accomplished by using phase locked loop circuitry on each input to keep them in synchronization with each other. This approach is cumbersome, in terms of physical space required for all the components, and costly, in terms of component cost. This problem escalates rapidly when it is desired to switch between several reference input signals since, for each such input, an entire phase correction circuit is required